Ausferritic Wear-Resistant Steel Castings

ABSTRACT

A method for preparing an austempered, work-hardening steel casting. A melt with certain chemical ranges including relatively high carbon and silicon content is poured, heat treated, cooled, austenitized, quenched and austempered, before final cooling to room temperature. This process provides a steel casting with increased wear life, characterized by a duplex microstructure containing ferrite plus carbon in solution in austenite known as ausferrite. The austenite will transform during abrasive service to a hard, wear-resistant martensite on the surface.

PRIORITY CLAIM

This application claims priority to and the benefit of provisionalpatent application No. 61/235,592, filed Aug. 20, 2009, and entitledAusferritic Wear-Resistant Steel Castings.

FIELD OF INVENTION

The present invention is in the technical field of material science.More particularly, the present invention is in the technical field ofwear resistant cast or forged steel.

BACKGROUND

During the manufacturing of wear resistant steels produced using thesteel casting sand molding process, limitations exist that affect theoverall wear life of the final product. These limitations are due to thehardness that can be achieved using conventional processing and heattreating methods. With low alloy steels, a relationship exists betweencarbon and hardness such that increasing the carbon content of the steelproduces higher final hardness and greater wear resistance. However, asthe carbon content increases, the steel becomes more brittle, and therisk of cracking during the manufacturing processor during useincreases. These factors limit the hardness, toughness, and wear life ofthe product.

Conventional heat treatment of carbon low alloy material to achievemaximum hardness consists of converting austenite to martensite by rapidwater quenching. See FIG. 1, Cooling Rate Curve A. This transformationincreases stresses in the part due to a volumetric phase change as wellas geometry-related factors which increase in proportion to the carboncontent. Therefore, it would be desirable to manufacture steel castingswith a chemistry and heat treatment that would allow much higher carboncontent without risk of cracking, resulting in higher hardness andlonger wear life.

The present invention satisfies these needs by providing a method forproduction of a cast or forged steel product with increased wear life,characterized by a duplex microstructure containing ferrite plus carbonin solution in austenite known as ausferrite. The austenite willtransform during abrasive service to a hard, wear-resistant martensiteon the surface.

SUMMARY

One embodiment of the present invention comprises a method of preparingan austempered, work-hardening steel casting, comprising melting analloy mixture containing (i) from about 0.6 to about 1.0 percent byweight carbon; (ii) from about 0.5 to about 1.0 percent by weightmanganese; (iii) from about 1.5 to about 2.5 percent by weight silicon;and (iv) the remainder iron. If the casting is greater than aboutone-inch thick, then it is advantageous to add at least one hardeningagent to the melt, selected from the group consisting of from about 0.4to about 2.5 percent by weight chrome; from about 0.0 to about 0.75percent by weight nickel; or from about 0.0 to about 0.5 percent byweight molybdenum. After the alloy is melted, it is poured into a moldof desired shape at a temperature of about 2700°-2800° F. and cooled inthe mold until it reaches a temperature of less than about 1000° F.,followed by shake-out and cooling to ambient temperature. Then, thecasting is austenitized at a temperature of about 1600°-1800° F. untilan austenitic matrix is achieved. Next, the casting is quenched in amedium capable of providing a quench rate sufficient to transform theentire casting to the desired ausferric matrix to a temperature of about550°-725° F., and that temperature is maintained for about 1-6 hours.The resulting ausferric matrix casting is then cooled to ambienttemperature. It is preferable, between the melting and pouring steps todeoxidize the alloy mixture by adding aluminum in the range of 0.02-0.04percent by weight. It is also preferable between the first cooling stepand the austenitizing step to homogenize the casting by heat treating ata temperature of about at least 1800° F. for a sufficient length of timeto reduce the number and size primary carbides. This time usually is aminimum of about 30 minutes per inch of section thickness, followed byair cooling to ambient temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be explained, by way of example only, withreference to certain embodiments and the attached Figures, in which:

FIG. 1 is a time-temperature-transformation diagram of a typical, priorart method of producing a steel casting;

FIG. 2 is a time-temperature-transformation diagram of one embodiment ofthe method of the present invention; and

FIG. 3 is a time-temperature process chart of one embodiment of themethod of the present invention.

DETAILED DESCRIPTION

In one embodiment of the present invention, steel castings are producedwithin the chemical ranges shown in Table 1 below. Specifically, a highcarbon and silicon content are used to produce the desired workhardening results. In heavier sections, the formation of uppertransformation ferrite and carbides is prevented by enhancing thehardenability of the chemistry with the addition of hardenabilityagents, such as chromium, molybdenum, nickel or combinations of these,in combination with the specialized austempering heat treatment.

TABLE 1 Element Percent by Weight Carbon 0.6-1.0 Manganese 0.5-1.0Silicon 1.5-2.5 Chrome 0.4-2.5 Nickel 0.0-0.75 Molybdenum 0.0-0.5 IronRemainder

Depending on casting size, it may be necessary to add hardenabilityagents, such as molybdenum, chrome, or nickel, either singly or in anycombination thereof, for aiding austemperability. For example, a smallcasting up to an inch thick usually does not require alloying with theabove mentioned hardenability agent(s) because the part is so small thatsufficient quenching severity can be experienced throughout the bulk ofthe casting without the hardening agent(s). On the other hand, heaviercastings (generally greater than about one-inch thick) usually requirethe addition of such agents to allow through quenching of the thickercasting components to achieve through hardening of the desired severity.Ranges of the hardenability agents change with casting size, but rangesof molybdenum 0-0.5% by weight, chrome 0.4-2.5% by weight, and nickel0-3.5% by weight have been found particularly effective for largercastings having thicknesses greater that one inch. The amount of theseelements to be incorporated is dependent upon the quenching equipmentand the quenching mediums being used, as will be understood by thoseskilled in the art.

As shown in FIG. 3, preparing austempered, work hardening steel castingsin accordance with one embodiment of the present invention comprises (a)melting the cast steel mixture to form a melt and preferably deoxidizingthe steel by adding aluminum in the range of about 0.02-0.04% by weightto reduce harmful oxygen and to minimize grain growth; (b) pouring themelt into a mold of desired shape at a temperature of about 2700°-2800°F. (depending on the exact chemistry and section thickness) to form anear net shape casting; (c) allowing the casting to cool in the molduntil it reaches a temperature less than about 1000° F., and thenshakeout the casting and continue cooling to ambient temperature (i.e.,below about 90° F.); d) preferably homogenizing the castings by heattreating at about 1800° F. minimum for a sufficient length of time toreduce the number and size of primary carbides, followed by cooling toambient temperature; (e) austenitizing the casting at a temperature ofabout 1600°-1800° F. until an austenitic matrix is achieved; (f)quenching the austenitic casting to a temperature of about 550°-725° F.(and maintaining that temperature preferably between about 1 to 6 hoursdepending on section thickness, chemistry, and desired properties) in amedium such as, but not limited to, molten salt or a medium capable ofproviding a quench rate sufficient to transform the entire casting tothe desired ausferritic matrix; and (g) cooling the ausferritic matrixcasting to ambient temperature (e.g., below about 90° F.).

As shown in FIG. 2, heat treatment consists of first homogenizing at atemperature of 1800° F. minimum (with 1850°-1900° F. being preferred)for a sufficient length of time to help dissolve primary carbides formedduring the casting process (see area “a” on FIG. 2). As a practicalmatter, all (or in some cases even the majority) of primary carbidescannot or need not be dissolved given temporal process constraints,chemistry, and desired material properties. However, the homogenizationreduces the number and size of primary carbides, resulting in anincreased amount of carbon in the microstructure during the remainingprocess steps. The homogenization time is normally about 30 minutes perinch of section thickness, followed by air cooling to ambienttemperature prior to normal steel foundry cleaning operations. Aftercleaning, austenitize at a temperature of 1600°-1800° F. (see area “b”),then holding until the entire casting reaches temperature in order toachieve an austenitic matrix. The specific temperature and hold time isdetermined by the final chemistry and section thickness, and a range of1650°-1750° F. is preferred. Then austempering by quenching in a mediumsuch as, but not limited to, molten salt or a medium capable ofproviding a quench rate sufficient to transform the entire casting tothe desired ausferritic matrix, at a temperature of about 550°-750° F.(with about 650°-725° F. being preferred) and maintaining thattemperature usually 1 to 6 hours depending on section thickness,chemistry, and desired properties (with a range of 2 to 4 hours beingpreferred), until the casting is substantially transformed to anausferritic matrix, as shown by line segment “cd” in FIG. 2. The castingis then cooled to ambient temperature as indicated by line segment “de”in FIG. 2.

As shown at line segment “cd” on FIG. 2, during the nucleation andgrowth phase of austempering a silicon alloyed cast steel at 550°-725°F., austenite directly decomposes into a carbide-free ausferritemicrostructure. Due to its effect on carbide formation, high silicon notonly stabilizes a carbon saturated austenite in the austemperedstructure but also retards the formation of unwanted carbides. This ischaracteristic of the ausferrite microstructure, which consists of acarbide free blend of acicular ferrite and austenite. Ausferrite has acombination of high strength and toughness; also, due to the presence ofcarbon saturated austenite, it has the ability to work harden on thesurface under low and high stress abrasive environments. Deformationinduced by high-impact, abrasive wear or sliding abrasion converts tofine martensite thus producing a surface with a much higher convertedhardness (about 600+Brinell) resulting in excellent wear-resistancequalities.

In one preferred embodiment, for a casting having a three-inchthickness, the following specific chemistry was found to produce awear-resistant steel in accordance with the present invention: carbon0.9% by weight, manganese 0.6% by weight, silicon 2.0% by weight, chrome0.6% by weight, nickel 0.25% by weight, and molybdenum 0.25% by weight.For this chemistry, forty to fifty-percent improvements in wearresistance, as compared to a conventional wear-resistant alloy, havebeen achieved. The specific chemical elements vary from the foregoingspecifications within the ranges provided of Table 1, depending oncasting size, thickness and effectiveness of the quenching medium; aswill be understood by those skilled in the art.

Another embodiment comprises using the principles the present inventionwith the forging process, as follows: (a) melting the cast steel mixtureas described by Table 1 to form a melt; (b) pouring the melt into a moldto form a billet or forging blank, at a temperature of about 2600°-2800°F.; (c) after shakeout, allowing the billet to cool to ambienttemperature for normal cleaning operations; (d) re-heating the billet tothe desired forging temperature as required by the forging processnormally in the range of 2000°-2400° F.; (e) forging to near a desirednet shape; (f) austempering as shown in FIG. 2 by first austenitizingthe forging at a temperature of 1600°-1800° F. and holding the forgingat temperature until an austenitic matrix is achieved, the specifictemperature and hold time being determined by final chemistry andsection thickness; (g) quenching the austenitic forging to a temperatureof about 550°-725° F. (and maintaining that temperature usually about1-6 hours depending on section thickness, chemistry, and desiredproperties) in a medium such as, but not limited to, molten salt totransform the entire casting to a desired ausferritic matrix; and (h)cooling the ausferritic matrix forging to ambient temperature.

Therefore, utilizing the combination of the steel casting sand moldingor forging processes, a chemistry as defined by Table 1 consisting ofhigh carbon, high silicon chromium molybdenum steel along with theaustempered heat treatment as shown in FIG. 2, a superior higherhardness, work-hardening, wear-resistant material can be producedwithout the risk of cracking or distortion as compared to conventionalmethods.

Although the present invention has been described and shown withreference to certain preferred embodiments thereof, other embodimentsare possible. The foregoing description is therefore considered in allrespects to be illustrative and not restrictive. Therefore, the presentinvention should be defined with reference to the claims and theirequivalents, and the spirit and scope of the claims should not belimited to the description of the preferred embodiments containedherein.

1. A method of preparing an austempered, work-hardening steel casting,comprising: (a) melting an alloy mixture containing (i) from about 0.6to about 1.0 percent by weight carbon; (ii) from about 0.5 to about 1.0percent by weight manganese; (iii) from about 1.5 to about 2.5 percentby weight silicon; and (iv) the remainder iron; (b) pouring the meltinto a mold of desired shape at a temperature of about 2700°-2800° F.;(c) allowing the casting to cool in the mold until it reaches atemperature of less than about 1000° F., then shaking out said castingand allowing it to cool to ambient temperature; (d) austenitizing thecasting at a temperature of about 1600°-1800° F. until an austeniticmatrix is achieved; (e) quenching the austenitic casting to atemperature of about 550°-725° F. and maintain that temperature betweenabout 1-6 hours in a medium capable of providing a quench ratesufficient to transform the entire casting to the desired ausferricmatrix; and (f) cooling said ausferric matrix casting to ambienttemperature.
 2. The method of claim 1, wherein said casting is greaterthan about one-inch thick, and wherein said alloy mixture melted in step(a) further comprises at least one hardening agent selected from thegroup consisting of: (v) from about 0.4 to about 2.5 percent by weightchrome; (vi) from about 0.0 to about 0.75 percent by weight nickel; or(vii) from about 0.0 to about 0.5 percent by weight molybdenum.
 3. Themethod of claim 1, further comprising, between said melting step (a) andsaid pouring step (b), deoxidizing said mixture by adding aluminum inthe range of 0.02-0.04 percent by weight.
 4. The method of claim 1,further comprising, between said cooling step (c) and said austenitizingstep (d), homogenizing said casting by heat treating at a temperature ofabout at least 1800° F. for a sufficient length of time to reduce thenumber and size primary carbides.
 5. The method of claim 4, wherein saidhomogenizing comprises holding the castings at said temperature for aminimum of about 30 minutes per inch of section thickness, followed byair cooling to ambient temperature.
 6. The method of claim 5, whereinsaid temperature is about 1850°-1900° F.
 7. The method of claim 1,wherein in said austenitizing step (d), said temperature is about1650°-1750° F.
 8. The method of claim 1, wherein the quenching medium insaid quenching step (e) is molten salt.
 9. The method of claim 1,wherein in said quenching step, said temperature is about 650°-725° F.10. The method of claim 1, wherein in said quenching step, said castingis held at said temperature for about 2 to 4 hours.
 11. The method ofclaim 1, wherein said casting is about three inches thick, and whereinin said melting step (a), said alloy mixture contains 0.9 percent carbonby weight, 0.6 percent manganese by weight, 2.0 percent silicon byweight, and further comprises 0.6 percent chrome by weight, 0.25 percentnickel by weight, and 0.25 percent molybdenum by weight.
 12. A method ofpreparing an austempered, work-hardening steel forging, comprising: (a)melting an alloy mixture containing (i) from about 0.6 to about 1.0percent by weight carbon; (ii) from about 0.5 to about 1.0 percent byweight manganese; (iii) from about 1.5 to about 2.5 percent by weightsilicon; and (iv) the remainder iron; (b) pouring the melt into a moldto form a billet at a temperature of about 2600°-2800° F.; (c) allowingthe billet to cool in the mold until it reaches a temperature of lessthan about 1000° F., then shaking out said casting and allowing it tocool to room temperature; (d) heating the billet to the desired forgingtemperature as required by the forging process; (e) forging the billetto near a desired shape; (f) austenitizing the casting at a temperatureof about 1600°-1800° F. and holding the casting at said temperatureuntil an austenitic matrix is achieved; (g) quenching the austeniticcasting to a temperature of about 550°-725° F. and maintaining thattemperature for about 1-6 hours in a medium capable of providing aquench rate sufficient to transform the entire casting to a desiredausferric matrix; and (h) cooling said ausferric matrix casting toambient temperature.
 13. The method of claim 12, wherein said forging isgreater than about one-inch thick, and wherein said alloy mixture meltedin step (a) further comprises at least one hardening agent selected fromthe group consisting of: (v) from about 0.4 to about 2.5 percent byweight chrome; (vi) from about 0.0 to about 0.75 percent by weightnickel; or (vii) from about 0.0 to about 0.5 percent by weightmolybdenum.
 14. The method of claim 12, wherein in said heating step(d), said forging temperature is about 2000°-2400° F.
 15. An articlemade of a steel alloy cast in accordance with the method of claim
 1. 16.An article made of a steel alloy forged in accordance with the method ofclaim 12.